1. Field of the Invention
The present invention relates to antennas. More particularly, it relates to three-dimensional, volumetrically compact antennas of high efficiency. It also relates to compact loop antennas of high efficiency, high gain, and the ability to be tuned over a very wide frequency range.
2. Prior Art
There are various types of antennas that are known in the art. These antennas include the full-wave loop, which can be of any-shaped perimeter from a triangle, square, rectangle, to a polygon with “n”-sides, culminating in a circle. Other such antennas involve operating the above loops at fractions of their resonant frequencies and are therefore small in size with respect to wavelength.
FIG. 1 illustrates one such prior art antenna; a symmetrical octagonal loop 20, which can be full-sized or compact in size depending on the frequency of operation.
Such loop antennas can be fed anywhere on their perimeter, such as at a feed point 22 where the loop 20 is discontinuous. If fed, with respect to ground, at their lower-mid or top-midpoints, the resultant radiation is horizontal in polarization. If fed ninety electrical degrees from either of these points, they are vertically polarized. They may be fed at any other point and the result is mixed polarization.
When these loops are resonant, at a perimeter that is nominally equal to one wavelength, they have radiation resistances (Rrad) in the range of 120 ohms. If, however, such symmetrical loops are operated at frequencies below those where the perimeter is a half-wavelength, they have very low radiation resistance (Rrad) and inductive reactance (XL). These are the most commonly-used compact antennas and are called “Compact Loops” or “Magnetic Loops”.
In general the shortcomings of simple Compact Loops, including low radiation resistance and low gain, are well known. Very low feed resistance Rin (the sum of Rrad and Rloss) must be stepped up by various matching networks in order to enable the antenna to be matched to the common 50 ohm standard. The present state-of-the art with respect to such Compact Loops is as follows:
A compact antenna is one that is a small fraction of a wavelength in size. There is no definition of what “compact” means but the most common form of such an antenna is the simple planar circular or octagonal loop having a circumference from 0.03 to over 0.1 wavelength and a diameter of circumference/Π.
These small loops operate on the principle that they are inductive at perimeters which are small fractions of a wavelength. This makes them amenable to tuning to resonance with high-Q capacitors rather than lossy inductors. Capacitors may be used in series with the feed (Cseries) to tune out the inductive reactance. Capacitors may also be used in “T” or gamma matching systems as part of a matching network.
In order to best illustrate the utility of the novel volumetrically compact and efficient antennas embodied in this invention it is necessary to examine in detail the operating parameters of these existing compact loops.
As a basis of comparison, a simple Compact Loop as in FIG. 1, of 1.3 m diameter and with a perimeter of 4.08 m is examined. It is fed at one side that is orthogonal to the ground and is vertically polarized. At a frequency of 7 MHz, the perimeter is just under 0.1 λ. The loop is composed of ¾″ or 19 mm thick copper rod, or wire or tubing and is placed 1 meter above “average” ground. All of the loop antennas to be discussed herein are of this size, composed of this diameter rod or wire or tubing and modeled at the same height above ground.
When this loop is modeled, it has a Rin of 0.12 ohms and this is, in turn, composed of a Rloss of 0.05 ohms and a Rrad of 0.07 ohms. The modeled gain is minus 4.51 dBi (−4.51). The input impedance has a reactive component of 156 ohms and this must be tuned out via a capacitor in series with the feedpoint (Cseries).
The above-quoted gain figure involves no losses beyond the copper wire of which it is composed. Due to the extremely low Rrad of 0.07 ohms, the introduction of even 0.1 ohm loss as with a tuning capacitor lowers the gain to −7.1 dBi. Further losses in construction, amounting to only 0.4 ohms (for a total added Rloss of only 0.5 ohm) lowers the gain to −11.6 dBi. An impedance matching network—necessary to step up (500:1) the Rin from fractional ohms to match a source of 50 ohms—can be conservatively estimated to add an additional loss of gain of about 3 dB.
Thus, as discussed in general above, the final gain of this loop can therefore be in the range of −10 to −14.6 dBi depending on the quality of components and construction techniques.
In addition to the low gain the radiation patterns are such that, when operated vertically polarized and near ground level, most of the radiation in the elevation lobe is near the zenith and not near the horizon. The low gains at low radiation angles render these poor antennas for long distance communication on a reliable basis and susceptible to high-arrival-angle interference.
There are other types of loops. As noted above, full-wave loops can be made with any-shaped perimeter. The loop configuration most amenable to the three-dimensional manipulation that results in a volumetrically small antenna is the rectangular loop or rectangle.
FIG. 2 illustrates an antenna in the shape of a rectangle 24, the characteristics of which are such that the fed or radiating wires 26 and 28 need not be the same size as the orthogonal wires 27 and 29 connecting them at their ends. The antenna in FIG. 2 has a pair of vertically oriented radiating elements, with one of the pair being fed at a feed point (not shown). The radiating elements are shorter than the orthogonal wires connecting them. As the radiating wires are made shorter and the length of the orthogonal wires between them increases to maintain a full-wavelength perimeter, the Rrad decreases and the gain increases. The Rrad is proportional to the size of the radiating elements and the gain is a function of the separation between them.
When resonant, such a full-wave loop's currents in its radiating wires are codirectional and in phase with each other and contribute to directivity. Additionally, the interconnecting orthogonal wires, horizontal in the case of the illustration, have out-of-phase currents with the resultant cancellation of radiation in the horizontal plane.
The applicant is aware of three antennas having a superficial similarity in form to the antennas in this application. These are described in:
U.S. Pat. No. 4,358,769, issued to Tada et al., for Loop Antenna Apparatus With Variable Directivity. This antenna is not compact and is composed of a plurality of relatively large loops with a plurality of ports. It is designed to enable changing the directivity and gain to maximize signal strength in the desired direction.
U.S. Pat. No. 5,258,766, was issued to Murdoch or Antenna Structure For Providing A Uniform Field. This antenna is not compact and is not composed of enclosed loops. It is composed of a plurality of partial loops and a plurality of ports and is designed to produce a three-dimensional electric field and involves a complex feed system in order to insure three-dimensional radiation.
U.S. Pat. No. 6,400,337, to Handelsman, the present inventor, for Three-dimensional Polygon Antennas. These antennas consist of a plurality of three or more appended full-wave loops that are arranged three-dimensionally. Each of a plurality of radiating elements is fed from a common source and the design goal is a very large bandwidth; in the order of two or more octaves. These are volumetrically large and function similarly to dipoles of very large diameter. The feed system is the basis for these antennas' bandwidth performance.